Microlithographic projection exposure apparatuses are used to transfer structures arranged on a mask onto a photosensitive layer, for example a photoresist or the like. For this purpose, the projection exposure apparatus includes an illumination device having a light source and an illumination system, which conditions the projection light generated by the light source and directs the conditioned light onto the mask. The mask illuminated by the illumination device is imaged onto the photosensitive layer by a projection objective.
When the wavelength of the projection light is shorter, commensurately smaller structures can be defined on the photosensitive layer with the aid of the projection exposure apparatus. For this reason, increasing use is now being made of projection light in the extreme ultraviolet spectral range, i.e. so-called EUV radiation, the central wavelength of which is at 13.5 nm. Such projection exposure apparatuses are often referred to for short as EUV projection exposure apparatuses.
Because, in general, there are no optical materials which have a sufficiently high transmissivity for such short wavelengths, an EUV projection exposure apparatus usually includes reflective optical elements in the form of mirrors. The mirrors are arranged in the illumination device of the EUV projection exposure apparatus, and the mirrors are used to guide the light onto the mask to illuminate the mask. With the aid of the mirrors of the associated projection objective, the illuminated mask is correspondingly imaged onto the photosensitive layer.
In order to carry this out with the desired accuracy, the mirrors are aligned precisely in all six degrees of freedom.
For precise alignment of mirrors in a projection objective, among other things, hexapod systems are known which operate according to different principles.
Thus, for example, hexapod systems are known which have a baseplate as a carrying structure which can be adjusted via replaceable spacer elements. With these known hexapods, to replace a spacer element and align the mirror carried by the hexapod, the mirror is first separated from the supporting structures and removed from the hexapod. After particular spacer elements have been replaced, the mirror is reconnected to the supporting structures of the hexapod. However, forces are exerted on the components involved, so that the alignment of the mirror is changed from the desired target alignment relative to the hexapod. If appropriate, this is corrected by readjustment which may, in turn, be subject to these perturbing influences.
Other known hexapod systems operate according to another principle and include solid-state articulations. Examples are described, for example, in EP 1 632 799 B1 or DE 10 2009 044 957 A1. Such hexapods include six supporting structures in the form of supporting arms, by which the optical element is carried and which cooperate in parallel kinematics. Two supporting arms in this case respectively form a bipod unit. In EP 1 632 799 B1, one coupling end of a supporting arm can move so that the angle between the optical element and the supporting arm in question changes and the position of the optical element is modified. The working length of the supporting arm, which was mentioned in the introduction, in this case remains unchanged. In DE 10 2009 044 957 A1, the supporting arms are formed as flexural elements. When such a flexural element bends, its working length shortens.
The carrying structure is generally installed in a stationary manner in the housing of the illumination device, and may also be formed by the housing or a frame of the illumination device itself.
The interior of an illumination device is usually evacuated, typically to attain a high vacuum. For this reason—and in principle when an illumination device is used in a semiconductor clean room—the use of a drive mechanism for changing the position of a mirror (for example, in the form of actuators, micrometer screws or differential thread apparatus) is not possible or only very limitedly possible. For actuators, elaborate encapsulations are generally used to prevent degassing of actuator materials.
In the course of the operation of an illumination device, it may furthermore occur that a mirror is removed repeatedly from the illumination system and replaced by another mirror, before being remounted at a given time. Besides precise alignment of the mirrors per se, a particularly important aspect when using mirrors in an illumination device is then the reproducibility of the position and alignment of the mirror, even if for a certain time it is not used and has been temporarily stored in a storage place. When this mirror is reinstalled, it is fully positioned and aligned again. The reproducibility of the position and alignment of a mirror after its storage and refitting into the illumination device is, for known systems, in the range of up to 10 μm, so that the realignment of the mirror is comparatively elaborate.
The removal and refitting of a mirror may, however, lead to insufficiently calculable position and situation displacements of the mirror relative to the carrying structure, which impairs the accuracy of its alignment. Here, the fastening mechanism by which the mirror or the carrying structure is coupled to the supporting structure is an important factor.
Overall, the desired accuracy for the alignment of a mirror of an illumination device for microlithographic EUV projection exposure apparatuses is increasing constantly from year to year. The position and alignment of each mirror in the illumination device should nowadays preferably be adjustable with tolerances of merely 2 μm to 3 μm, or up to 7 μrad per degree of freedom.